RF television signals may have carriers with non-standard frequencies near respective standard (i.e., "broadcast") frequencies. Such non-standard frequency RF television signals may be provided by cable distribution networks or television accessories such as a VCR, video disk player, video camera or video game. Accordingly, many tuning systems include search provisions which change the frequency of the local oscillator signal in a step-wise search in the vicinity of the nominal local oscillator frequency for a newly selected channel until correct tuning is achieved. It is desirable that the tuning system be relatively stable after a new channel is selected or after a new step is taken so that the output signals of the detectors utilized for the search can be relied on.
One method of determining that a tuning system has achieved stability after a new channel has been selected or after a new step is taken in a search is to provide a time delay corresponding to the time required for stability to be achieved. With regard to channel changes, the required time delay varies with the "spacing" between the previous channel and the new channel. That is, it takes longer for a tuning system to make a channel change between channels at opposite ends of a tuning band (e.g., channels 2 and 6 in the low VHF television band) than it does to make a channel change between adjacent channels (e.g., channels 2 and 3). Accordingly, the time delay is usually set to the longest required time delay. Considering that the display of a television system is often "blanked" so as to prevent the display of transient effects during maximum time required for any channel change operation, including any search, e.g., in the order of 0.5 seconds, the apparent tuning time which the user is exposed to may be much greater than the actual tuning time required for a particular channel change operation. This irritating problem is multiplied when a user used "channel up" and "channel down" scanning buttons which are conventionally provided to allow the user to browse through various channels in sequence.
Phase locked loop (PLL) tuning systems sometimes include a so-called "lock" detector for determining when the tuning process has achieved substantial stability. A lock detector examines an error signal including pulses representing the phase and frequency differences between the local signal and a reference frequency signal, and generates a "lock" signal when the pulses of the error signal have become narrower than a predetermined width.
Most present PLL tuning systems are to a large extent incorporated within an integrated circuit (IC). While many PLL ICs are available, many do not include a lock detector. For example, a lock detector may not be provided if the PLL is incorporated in a control IC which is intended to control many other functions of a television system, in addition to tuning control, and IC (or "chip") area is at a premium. A lock detector may also not be provided in a PLL IC dedicated to tuning control because of the desire to save external connection terminals as well as chip area.
Thus, there is a need for a method and apparatus for detecting when a tuning system, which may not include a lock detector, has achieved stability and which does not require the same maximum time delay for every tuning change.